1. Field of the Invention
The present invention relates to an over-current detecting circuit (thereafter, xe2x80x9cover-current detecting circuitxe2x80x9d is abbreviate to xe2x80x9cOCD Circuitxe2x80x9d.) for detecting the over-current state of an output transistor having an open drain structure, and a semiconductor integrated circuit (SI Circuit) having the OCD Circuit built-in.
2. Description of the Related Art
In many cases, SI Circuits which drive a solenoid or the like have therein an output transistor having an open drain structure. In such SI Circuits, a large current passes through the output transistor which frequently destroys the output transistor when an external load such as either a solenoid suffers catastrophic failure or the state of an external power source varies abnormally. In order to prevent such failure, it is necessary to have a mechanism for monitoring the amount of current that passes through the output transistor at all times and immediately sever the current towards the output transistor when detected that the current is in an over-current state. A circuit including such a mechanism is called an OCD Circuit. The following will describe a conventional SI Circuit having this mechanism built-in.
FIG. 1 is a conventional view showing an SI Circuit disclosed in Japanese Patent Application Laid-Open No. 2-87817.
An SI Circuit 70, disclosed in the above-mentioned publication, is a device for controlling whether or not a driving current is supplied to the load RLxe2x80x2 which connects an output terminal 73 and the external power source VBxe2x80x2. And the SI Circuit 70 operates in accordance with an input signal supplied to the input terminal 76 from the outside. Whether or not the driving current is supplied to the load RLxe2x80x2 is decided by whether output transistor Q71, which has an open drain structure, is in a state of conduction or non-conduction. The output transistor Q71 is controlled by a logic circuit 75, which receives the input signal. That is, if the logic circuit 75 outputs a high-level signal in response to the input signal, the output transistor Q71 becomes conductive, and consequently passes the driving current through the load RLxe2x80x2. On the other hand, if the logic circuit 75 outputs a low-level signal in response to the input signal, the output transistor Q71 becomes nonconductive, thus severing the supply of the driving current toward the load RLxe2x80x2.
However as previously described, in the case that either the load RLxe2x80x2 short-circuits or an abnormally high voltage is generated in the external power source VBxe2x80x2, an unexpected large current is sent in the conductive state to output transistor Q71 and it may be destroyed. To prevent this, the SI Circuit 70 has an OCD Circuit, comprised of reference voltage generating circuit (RVG Circuit) 71, resistors R75 and R76, diodes D71-D7n, and a comparator 72. Such an OCD Circuit is a circuit for detecting an over-current state of the transistor Q71 and for comparing the voltage at the output terminal 73 with a reference voltage Vrxe2x80x2.
The reference voltage Vrxe2x80x2 for detecting the over-current, can be obtained by dividing the output voltage from the RVG Circuit 71 through a voltage divider wherein the resistors R75 and R76 and n diodes D71-D7n are connected in series. The comparator 72 continually compares the reference voltage Vrxe2x80x2 with the output voltage from the output transistor Q71, so that whenever the detection of an over-current is found on the basis of an abnormal rise in the output voltage from the output transistor Q71. The over-current detecting terminal 74 outputs an over-current detecting signal. The over-current detecting signal is fed back to the logic circuit 75 to trigger the output transistor Q71 into an OFF-state. As a result, the output transistor Q71 is protected from destruction based on an over-current.
The diodes D71-D7n are set to compensate the temperature characteristic of the ON-state resistance of the output transistor Q71. For this reason, any over-current is detected under good dependency at any temperature.
An output voltage VBGxe2x80x2 from the RVG Circuit 71 is also used for detecting temperature by a temperature detecting circuit. Therefore, dependency, (contingent of the output voltage VBGxe2x80x2 from the RVG Circuit 71) of the temperature must be small. It is impossible to cause the output voltage VBGxe2x80x2, itself, of the RVG Circuit 71 to have a temperature characteristic for compensating the temperature characteristic of the output transistor Q71. For this reason, in order to perform over-current-detection with good dependency upon temperature, the diodes in series are used to generate the reference voltage Vrxe2x80x2, which has the same temperature characteristic as the output transistor Q71 by using the output voltage VBGxe2x80x2 having no dependency upon temperature.
However, the reference voltage Vrxe2x80x2 for detecting an over-current is generated from a voltage divider to which the diodes D71-D7n and R75 and R76 are connected in series, causing a problem that the range wherein detection values thereof are set, is limited to a low range by a voltage-drop at the diodes connected in series.
For example, in the case that 5 diodes are used, the reference voltage Vrxe2x80x2 for detecting an over-current, is limited to lower than the output voltage VBGxe2x80x2 from the RVG Circuit 71, by the forward direction voltage reduced through the 5 diodes.
For this reason, the margin for setting the detection range is low. As the case may be, in order to obtain a desired reference voltage, it is necessary to sacrifice the temperature characteristic to some degree. Alternatively, in order to obtain a desired temperature characteristic, it is necessary to shift the reference voltage from a desired value by some degree.
The object of the present invention is to provide an OCD Circuit, making it possible to set the value for over-current detection to a desired value, while compensating the temperature characteristic of an output transistor sufficiently, with the SI Circuit having the detecting circuit.
According to one aspect of the present invention, an OCD Circuit is a circuit for comparing a voltage drop by an ON-state resistance of an output transistor with a reference voltage to detect an over-current state of the output transistor. The OCD Circuit comprises a first power source and an RVG Circuit which outputs a first reference voltage on the basis of a voltage supplied from the first power source. The OCD Circuit also comprises a constant-current source, which generates a constant current having a second temperature characteristic on the basis of the first reference voltage, and a current mirror circuit which inputs the constant current. Moreover, the OCD Circuit comprises a current-voltage converting circuit, which converts an output current from the current mirror circuit to a voltage then outputs a reference voltage having a temperature characteristic in proportion to the second temperature characteristic. A first temperature characteristic of the output transistor is compensated by the temperature characteristic of the reference voltage.
According to another aspect of the present invention, an OCD Circuit is a circuit for detecting an over-current passing through an output transistor which connects an output terminal and a power source. The OCD Circuit comprises a constant-current source which generates a first constant current having a given temperature characteristic on the basis of a reference voltage. The given temperature characteristic is substantially equal to a temperature characteristic of the output transistor. The OCD Circuit also comprises a current mirror circuit which generates a second constant current on the basis of the first constant current, and a current-voltage converting circuit which generates a reference voltage on the basis of the second constant current. Moreover the OCD Circuit comprises a comparator which compares the reference voltage with the voltage at the output terminal.
According to another aspect of the present invention, an SI Circuit comprises an output transistor which connects an output terminal and a power source, an OCD Circuit which detects a current passing through the output transistor, and a logic circuit which controls a conduction state of the output transistor on the basis of an input signal. The OCD Circuit has a constant-current source which generates a first constant current having a given temperature characteristic on the basis of a reference voltage. The given temperature characteristic is substantially equal to a temperature characteristic of the output transistor. The OCD Circuit also has a current mirror circuit which generates a second constant current on the basis of the first constant current, and a current-voltage converting circuit which generates a reference voltage on the basis of the second constant current. Moreover, the OCD Circuit has a comparator which compares the reference voltage with a voltage at the output terminal. The logic circuit causes the output transistor to be in a nonconductive state in response to generation of an output signal from the comparator, whether the input signal is inputted or not.